AC themostat calibration

I posted a question about my AC thermostat a week ago, the cooling was
working fine but heating was not. The heater will not turn on even when I
set it all the way to a low temperature (current temperature is say 70, I
set it to 60, it will not turn on, but it may two hours later...) I called
the AC company who installed it and they came out for a service fee of $100.
By the way this is the thermostat I am talking about, a real old one. It is
a White-Rodger mercury switch.
Front cover:

http://img.villagephotos.com/p/2005-10/1090906/inside.JPG
Back of front cover:

http://img.villagephotos.com/p/2005-10/1090906/coverback.JPG
Close up view of a sticker which should have the detail product information:

http://img.villagephotos.com/p/2005-10/1090906/productinfo.JPG
Close up view of the mercury bulb:

http://img.villagephotos.com/p/2005-10/1090906/mercurybulb.JPG
I am not sure exactly how things work. The technician told me that my AC
unit itself is working perfectly (LENNOX CB301-46-1P). However the
calibration is off. He says my problem is the dial probably adjusted too
far off. He showed me how to adjust the back cover dial to the current
temperature (he says it feels like 72 so he sets it to 72). After the
adjustment he left the cover off and showed me that if I push the bottom
dial to the left, the mercury bulb will tilt to the right and after 1 minute
the AC turns on. So the AC is working properly and the mercury is working
fine. He did tell me he can install a new digital thermostat for me the
unit itself will be $150 and installation $100, he did say the same unit
would be like $20 at HD and it can be easily done by myself.
So now, when I remove the cover and push the dial all the way to the left
and the mercury bulb tilted all the way to the right, the heat did not turn
on with settings set to AUTO and HEAT. After waiting like over an hour, it
finally turned itself on (for what reason I have no idea). The cooling side
worked fine, as soon as the bulb tilts right, there is a tiny spark and it
kicks on. The heater side did not. I have no clue why it worked perfectly
while he was here.
So now my option is to call him back (which means losing a whole day of
work - appointments are like 10AM - 3PM anytime in between...and pay $75),
buy a new thermostat myself and see, or play with the calibration a bit more
myself. But I don't think it's just calibration if I tilt the mercury bulb
all the way to the right and the heater does not turn on.
The only problem is with buying a new thermostat is to figure out which one
to buy. I browsed HD's catalog and they carried LUX and Honeywell? Any
specific models recommended? What should I watch out for when wiring them
besides turning off power? Is it a simple color coded 1 to 1 wiring or a
different model may require something different?
Thanks,
MC

So how much money are you willing to spend on this? You've already
been screwed once, ie, people that installed it charged you for a
service call. They saw you coming. quit screwing with the pos and get
a digital thermostat. The good ones are around 50.00 at the big box
stores. After you get it, go to the air handler portion of the unit
and unplug it. take the door off and write down the color of each
wire. standard wiring is r to r, g to g, c to c, w to w and y to y. If
it is not wired this way it is no big deal as long as you match the
same color at both ends. On the thermostat put a jumper between rc and
rh then put r to rh or rc, g to g, w to w, y to y and DO NOT hook up c
unless you have a two transformer system. If it is a two transformer
system then and only then, hook up c. The air handler MUST BE
UNPLUGGED OR YOU WILL SHORT A FUSE ON THE AIR HANDLER CIRCUIT BOARD.
After you get it all hooked up, plug the air handler back in and let
er rip.

If I understand the situation, either the tstat or the furnace is not
working correctly, since you continue to have problems even when the
mercury switch is clearly calling for heat. Perhaps there is some
corrosion (or similar problem) on the contacts on the heat/cool switch.
If you can see the guts of this switch, try pushing on the contacts
while the mercury switch is clearly calling for heat (or spray the
contacts with electrical contact cleaner (Radio Shack has this stuff).
Similarly, make sure all the screws holding down the wires are tight,
both at the stat and at the furnace.
Did the tech who came out confirm that the furnace was working
correctly? At this point, it's not clear, to me at least, whether the
problem is in the stat or in the furnace.
Al Moran wrote:

I have to agree with Al
I recommend picking up one at the hardware store. Ask the people there
about the differences in the different models and features they have. You
may be able to save some money on you heating and cooling by getting an
automatic set back type. They cost a little more but usually pay for them
selves quickly. If you have someone at home all day, then just get the
standard one. They come with instructions. They are easy to do yourself.

I vote to do it yourself! $250 is a lot of money for what should be a
30 minute job,if all goes right. If it doesn't,call the technician
-he'll probably charge double-just kidding ;) The advice in this group
sounds good from what I've seen in my short time here.

I don't recommend the LUX units. I have had problems
with a couple of them at one time and another.
Honeywell is a good brand. It will be more expensive
than the LUX. There are a couple of other good brands,
but I don't recall the names.
But do a little checking first. There way be some other
problem, from your description of what was happening.
I would try to figure out which 2 wires are connected
together when the mercury switch is closed, then go to
the furnace and CAREFULLY short those 2 wires where they
connect to the furnace control. If the furnace comes on
right away when the wires are shorted then the problem
is most likely in the themostat. Of course there is
some possibility that there may be a problem with the
wiring.
Bill Gill

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